The way we determine what's true or false, real or artificial, good or bad in movies tends to be highly individual. As a reviewer, I'm obliged to give movies star ratings, but they're simply a summary of my personal response, not a declaration of some objective value and certainly not of any sort of consensus. I was taken aback recently when I received a couple of e-mails from Star Wars fans asking how I could have concluded eight years ago that the "special edition" rerelease of that film was "worthless" when it gave so much pleasure to so many people. I might have given it an even lower rating if I could have, but all I meant by giving it no stars was that it was worthless to me. I'm not qualified to speak about its value to anyone else.
In my reviews I try to describe the paths that lead to my subjective response so that readers can decide whether some part of my path might be theirs too. In the case of Crash I may blanch at Haggis's narrative contrivances and think two stars, though I did enjoy them (three stars). But the vision of Los Angeles that they're designed to express strikes me as just and vital (four stars). So I wind up with an average of three. Viewers who find the vision uninteresting and the narrative contrivances acceptable but unenjoyable will come up with ratings of their own -- or arrive at the same rating for entirely different reasons.